Often cutting tools with indexable cutting inserts for the milling of right-angled corners in a workpiece have had cutting inserts of a rhombic or triangular shape. They have only two or three cutting edges but for a long time they were considered to be the only ones which make it possible to cut right-angular corners with sufficient side clearance. When a conventional right-angled indexable cutting insert is used for milling or turning of an internal side surface, it cannot be positioned with a radial or axial angle without its lower inner corner trailing on the generated surface. Hence, when cutting an inner 90.degree. shoulder it has been common practice to use either an insert with an acute angle between the main cutting edge and the secondary cutting edge or the parallel land, respectively, or also right-angled cutting inserts inserted into the tool body with a negative radial angle and a positive axial angle.
In U.S. Pat. No. 4,632,607 an indexable cutting insert is disclosed with a square basic shape for the milling of right-angled corners, which insert comprises four main cutting edges, thus realizing a substantially improved cutting economy. This is achieved by the fact that each main cutting edge has an appurtenant secondary cutting edge which is arranged outside the square basic shape upon a protrusion that sticks out in a direction mainly parallel to the appurtenant main cutting edge. This cutting insert works satisfactorily at a total cutting depth less than the length of the main cutting edge. However, if one intends to arrive at a total cutting depth that exceeds the length of the main cutting edge by performing several runs, then a protrusion will cause an undesired groove in the workpiece.
In U.S. Pat. No. 5,199,827 a further indexable cutting insert for milling is disclosed having a square basic shape. The insert is intended to be inclined axially positively and radially negatively in the milling cutter body. Sufficient axial clearance is obtained by inclining the insert radially negatively. Each flank surface is shaped with a planar relief surface, whose width increases in a direction towards one of the corners of the insert. Sufficient radial clearance between the parallel land and the main cutting edge is intended to be obtained by the angle between the parallel land and the adjacent main cutting edge of about 0 to 2.degree.. However, since the relatively short parallel land has to support considerable stress, it may become worn, whereby also the main cutting edge comes into contact with the generated surface on the workpiece, resulting in decreased surface smoothness, increased heat production and increased cutting forces.